1. Field of the Invention
The present invention relates to a deflection yoke for a picture tube of a projection color television receiver set, and more particularly to a deflection yoke for a picture tube for obliquely projecting light to a screen of a projection color television receiver set which has three picture tubes for emitting red light, green light and blue light, respectively, and reproduces a color image by projecting the lights on a common screen.
2. Description of the Prior Art
A projection color television receiver set has been known which has a picture tube having a phosphor for emitting red light, a picture tube having a phosphor for emitting green light and a picture tube having a phosphor for emitting blue light and projects images reproduced on face plates of the respective picture tubes onto a screen through projection lenses mounted in front of the respective face plates to reproduce a color image. In such a projection color television receiver set, the three picture tubes are usually arranged horizontally. For example, the picture tube for emitting the green light is arranged at a center and the picture tube for emitting the red light is arranged on one side while the picture tube for emitting the blue light is arranged on the other side. In this type of projection color television receiver set, the face plates of the picture tube for emitting the red light and the picture tube for emitting the blue light arranged on the opposite sides of the picture tube for emitting the green light are arranged obliquely to the screen so that the lights emitted therefrom are projected obliquely to the screen. Consequently, the images reproduced by the blue light and the red light are not reproduced in correct shape although the image reproduced by the picture tube for emitting the green light arranged at the center is reproduced in a relatively correct shape. As shown in FIG. 1, since a face plate 1G1 of a picture tube 1G is arranged in parallel to a screen 4, a rectangular raster (not shown) formed on the face plate 1G1 is enlarged by a lens 2G and projected on the screen 4 as an analogous shape of raster 5G as shown in FIG. 2. The raster on the face plate 1G1 is formed by deflecting electron beams emitted from an electron gun 1G3 by a deflection yoke 1G2. Since face plates 1B1 and 1R1 of picture tubes 1B and 1R are arranged obliquely to the screen 4 by an angle .theta., rectangular rasters formed on the face plates 1B1 and 1R1, when they are enlarged by lenses 2B and 2R and projected onto the screen 4, appear as trapezoidal rasters 5B and 5R as shown in FIG. 2. The angle .theta. is usually 5-10 degrees. When the picture tube is arranged on the right of the center 0 of the screen 4, the raster formed on the screen 4 is longer on the left hand and shorter on the right hand. When the picture tube is arranged to the left of the center 0 of the screen 4, the raster formed on the screen 4 is longer on the right hand and shorter on the left hand. In the projection color television receiver set, if the shapes of the rasters 5B and 5R are different, a misconvergence results.
In order for the rasters 5B and 5R to be formed in the correct rectangular shape on the screen 4, it is necessary that the rasters of complementary shape to those formed on the face plates 1B1 and 1R1 of the picture tubes 1B and 1R are preformed so that distortions of the rasters 5B and 5R are cancelled. For example, for the raster 5R which emits the red light, a raster 6 shown by a solid line in FIG. 3 must be formed on the face plate 1R. Horizontal lines 7a and 7b of the raster 6 include trapezoidal distortions and distances l.sub.1 and l.sub.2 of vertical lines 8a, 8b and 8c are different, that is, the distance l.sub.2 is longer than the distance l.sub.1. In the prior art projection color television receiver set, sub-deflection yokes are used to form the trapezoidal rasters on the face plates of the picture tubes and currents are supplied to the sub-deflection yokes from an electrical circuit. The electrical circuit is usually very complex in construction and a characteristic thereof changes as a power supply voltage changes. As a result, the shapes of the rasters formed on the face plates of the picture tubes change and hence the shapes of the rasters reproduced on the screen change, resulting in a misconvergence. In addition, linearity of the reproduced image also changes and a high quality of image is not reproduced on the screen.